This invention relates to the stabilization of clay formations, particularly those encountered in the drilling of and production from oil wells. More particularly, this invention is concerned with such stabilization by means of chemical agents.
Production of petroleum hydrocarbons is often troubled by the presence of clays and other fines capable of migrating in the formation. Normally, these fines, including the clays, are quiescent, causing no obstruction of flow to the well bore via the capillary system of the formation. However, when the fines are disturbed, they begin to migrate in the production stream and, too frequently, they encounter a constriction in the capillary where they bridge off and severely diminish the flow rate.
The agent that disturbs the quiescent fines is frequently the introduction of a water foreign to the formation. The foreign water is often fresh or relatively fresh water compared to the native formation brine. The water is frequently intentionally introduced for purposes of hydraulic fracturing of the formation rock to increase production rates. In any event, the change in the water can cause fines to disperse from their repository or come loose from adhesion to capillary walls.
Sometimes the loss of permeability is due to clay swelling with relatively fresh water without migration. But, often clay swelling is accompanied by migration of fines. Sometimes non-swelling clays can respond to the foreign water and begin to migrate. It is believed that swelling clays are the major mechanism of fines migration and/or swelling, because when formation cores are analyzed, the presence of swelling clays are an excellent indicator that the formation will be sensitive to foreign water intrusion, while the presence of non-swelling clays only is inconclusive.
Generally, swelling clays are in the smectic group including clay minerals such as montmorillonite, beidellite, nontronite, saponite, hectorite, and sauconite. Of these, montmorillonite is the clay mineral found most commonly in formation core analysis. Montmorillonite is commonly associated with clay minerals known as mixed-layer clays. Further information is contained in Jackson's Textbook of Lithology, pages 95 to 103.
Migrating fines including a host of clay and other minerals in minute particle size, for example, feldspars, fine silica, allophane, biotite, talc, illite, chlorite and the swelling clays themselves. Further information is contained in Theng's The Chemistry of Clay-Organic Reactions, pages 1 to 16.
Clays can also cause trouble in areas other than permeability reduction. When they are a component in shales, sandstones, or other formations, contact with a foreign water or at times with any water can cause the formation to lose strength or even disintegrate. This is a problem in building foundations, road beds, drilling wells, and any situation where the formation strength is important.
There have been numerous attempts to control the ill effects of water on clay and/or other fines. These have been principally in the oil industry. One idea is to convert the clay from the swelling sodium form or the more rare swelling lithium form to another cation form which does not swell as much.
Example cations that form relatively non-swelling clays are potassium, calcium, ammonium and hydrogen ions. When a solution of these cations, mixed or individually, flows past a clay mineral, they readily replace the sodium ion and the clay is transformed to a relatively non-swelling form (refer to Theng, Tables 2, 3, and 4). The use of acid, potassium, calcium, or ammonium ions to exchange for sodium ion has been successful in preventing damage to formations susceptible to plugging or disintegrating due to clays in their compositions.
One specific approach is that of U.S. Pat. No. 4,366,074 (McLaughlin - Halliburton, 1982) which teaches the use of a very wide variety of polymers, including poly(acrylamide-3-propyltrimethylammonium chloride): ##STR1## as clay stabilizers. While the illustrated compound is effective in shallow wells, it decomposes and loses its effectiveness at the higher temperatures encountered in deep wells.
WO 88/04680 (Meunier - Elf, 1988) teaches the use of quaternary salts of copolymers of an unsaturated acid or anhydride (including maleic anhydride) and another unsaturated compound (hydrocarbon, ester, or either), in a ratio of 1:1 to 1:4. While these materials are operable, they do not provide as high a degree of stabilization as is desired.
Accordingly, it would be desirable to provide a clay stabilization composition that would provide a high degree of stabilization and that would not decompose at the temperatures encountered in deep oil wells.